1. Field of the Invention
This invention relates to a method of exposing a peripheral part of wafer which is used for a fine pattern formation process in the production of ICs, LSIs and other electronics elements. More this particularly, this invention relates to removing, in a development step, an unnecessary portion of a photoresist coated on a semiconductor substrate, typically a silicon wafer, or a substrate consisting of a dielectric, a metal or an insulator.
2. Description of the Prior Art
In the manufacture of ICs and LSIs, for forming a fine circuit pattern, a photoresist pattern is formed by coating a photoresist on a silicon wafer or the like and exposing and developing the coated photoresist. The photoresist pattern thus formed is used as a mask to effect ion implantation, etching, lifting-off and other steps.
Usually, the photoresist is coated by spin coating. In the spin coating process, the wafer is spun while pouring photoresist onto the center position of the right side of the wafer. The poured photoresist is coated on the wafer by centrifugal forces. In this spin coating process, however, the photoresist is spun off a peripheral part of the wafer and is brought to the wrong side of the wafer.
FIG. 2 is a cross sectional view showing a photoresist coated on a wafer. In the Figure, reference numeral 1 designates a wafer, 1p a peripheral part of the wafer, 1a a photoresist portion on a pattern formation part of the wafer, 1b a photoresist portion on a peripheral part 1p, and 1c a photoresist portion brought to the wrong side of the wafer 1 from the edge thereof. The photoresist portion 1c brought to the wrong side of the wafer is not illuminated in an exposure step for pattern formation, and if it is a positive photoresist, it remains after the development.
FIG. 3 is a typical view showing a circuit pattern formed by exposure on the wafer. Each area labeled T corresponds to one circuit pattern. In a peripheral part of wafer, a correct circuit pattern can not be formed in many cases, or it can be formed with a inferior yield. The photoresist portion 1b on a peripheral part of wafer is substantially unnecessary in the circuit pattern formation. Even if the peripheral part of wafer is exposed by a stepper, the photoresist remains unnecessarily after, the development.
The remains of unnecessary photoresist coated on the peripheral part and brought to the wrong side of the wafer gives rise to the following problem. The wafer with the coated photoresist is conveyed in various steps and by various systems or units. A peripheral part of the wafer thus is sometimes mechanically chucked and sometimes rubs a wall of wafer cassette or like wafer accommodation means. In such cases, the unnecessary photoresist portion on the peripheral part of wafer is liable to be removed and re-attached to a pattern formation portion of the wafer. In this case, correct pattern formation can no longer be obtained, thus reducing the yield.
The fact that the unnecessary photoresist remaining on the peripheral part of wafer becomes "refuse" to reduce the yield poses significant problems particularly in view of the recent trend for higher functional levels and finer patterns in ICs.
For removing the unnecessary photoresist remaining on the peripheral part of wafer, a technique of removing the photoresist by a solvent spray method is in practical use.
In this method, the solvent is sprayed on the wrong side of the peripheral part of the wafer, solving the unnecessary photoresist. In this method, however, although the photoresist 1c as shown in FIG. 2 can be removed, the unnecessary photoresist 1b on the right side of the peripheral part of wafer can not be removed. Further, if it is arranged such that the solvent is sprayed on the right side of wafer 1 for removing the unnecessary photoresist 1b, not only does there arise a problem caused by scattering of the solvent, it is impossible to remove only the unnecessary photoresist with high controllability. Also, it is impossible to provide for a sharp borderline between the unnecessary photoresist 1b on the peripheral part of wafer and photoresist 1a on the pattern formation part of wafer which is necessary as a mask layer for a subsequent etching or an ion implantation process.
Recently, it has been in practice to expose the unnecessary photoresist on the peripheral part of wafer to remove it in the development process in addition to the exposure step for pattern formation of the wafer so called "photolithography".
In this peripheral part of the wafer exposure method, the borderline between the unnecessary photoresist 1b on the peripheral part of wafer and photoresist 1a necessary for the pattern formation as a mask layer for a subsequent ion implantation process or the like, as shown in FIG. 2, can be sharp and with high controllability, so that this method is superior to the solvent spray method.
In the above prior art of peripheral part exposure method, if the photoresist is irradiated with strong ultraviolet rays from the outset to shorten exposure time, gases that are generated by decomposition and evaporation of organic solvents contained in the photoresist or photochemical reaction of photoresist thereof are not emitted to the outside but becomes bubbles within the photoresist.
If such a bubble formation occurs, the portions with bubbles are spattered when the wafer rubs a wall of a wafer cassette or the like. The spattered photoresist is attached as "refuse" to the pattern formation part of the wafer to bring about the problem of pattern defects as noted above.